Day-to-day variability in voluntary wheel running among genetically differentiated lines of mice that vary in activity level

This study examined the day-to-day variability in voluntary wheel-running behavior among three genetically distinct lines of young male and female mice. Daily wheel revolutions were recorded at an age of 6–8 weeks in 10 males and 10 females from each of 3 lines: selectively bred line for high wheel running (Line 8), selectively bred for high wheel-running activity and fixed for a Mendelian recessive allele that reduces hind-limb muscle mass by 50% (Line 3), non-selected control (Line 2). There were significant mean differences in revolutions/day among weeks (P = 0.003), but the effect size was small (10%). Significant main effects for wheel running were also revealed for sex and line (P < 0.001). The grand mean ± SD for the coefficient of variation (CV) of intra-individual wheel running was 23.0 ± 10.8%. Although a significant main effect for the CV was found for week, the effect size was low (7%) (age 6 weeks, 23.4 ± 10.9%; age 7 weeks, 25.1 ± 13.2%; age 8 weeks, 20.1 ± 7.8%). The overall mean CV was similar between females (21.4 ± 9.8%) and males (24.4 ± 12.0%) and among lines (Line 2, 23.4 ± 9.8%; Line 3, 20.4 ± 7.6%; and Line 8, 25.0 ± 14.4%). These findings are consistent with our previous work in young humans and lend further support for the hypothesis that biological mechanisms influence daily levels of physical activity.